1. Field of the Invention
The present invention relates to a converter for driving light-emitting diodes, and particularly to a core spike-free converter, followed by an expanded flicker-free converter for driving light-emitting diodes.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a classic dc-dc converter 100 as an offline light-emitting diode (LED) driver.
In operation, a clock generator A3 enables R/S flip flop (R/S FF) A1 to drive a switch M1 at a fixed frequency. A comparator A4 detects current flowing through the switch M1 through a resistor R2 to reset the R/S FF. A leading-edge blanking A2 temporarily disconnects feedback signal in a predetermined time interval, preventing turn-on spikes of the switch M1 from resetting the R/S FF prematurely. The turn-on spikes are generally caused by a diode D5 and the switch M1.
The open-loop detects maximum switching current (flowing through the switch M1) to turn off the switch M1 (turn-on interval), and uses the correlations to estimate current flowing through LED strings of diode D6 for a given clock frequency.
A first capacitor C1 is a stabilization capacitor storing sufficient energy through a bridge rectifier composed of diodes D1, D2, D3, and D4, to continue normal operations between the line charges. The diode D5 is a flyback diode that provides current path to an inductor T1, to continue feeding a capacitor C2 and the LED strings of diode D6 when the switch M1 is off (turn-off interval).
The capacitor C2 and the LED strings of diode D6 can also be in series with the diode D5. The tradeoff is the larger inductor T1 to store higher energy to recharge the capacitor C2 during the turn-off interval of the switch M1 vs. the larger capacitor C2 since the switch M1 will run out of voltage headroom when voltage across the capacitor C1 falls below the dc voltage of the LED strings D6 during the turn-on interval. The tradeoff can be applied to all of the following embodiments of the present invention.
A resistor R1 provides startup of the classic dc-dc converter 100. An inductor T2, which is magnetically coupled to the inductor T1, provides a housekeeping voltage across a bus capacitor C3, and a diode D8 prevents the resistor R1 from being shorted out by the inductor T2 prior to the onset of oscillation. A Zener diode D11 regulates the housekeeping voltage. In some designs, the inductor T2 and the diode D8 are omitted if the current draw of all circuit blocks combined is at an acceptable level to be supplied by the resistor R1 alone, in the interest of cost over efficiency.
The classic dc-dc converter 100 works well under dc-dc conversion. However, in pursuit of a desirable high power factor (PF), the classic dc-dc converter 100 is often elevated to perform alternating current-direct current (ac-dc) conversion. As such, the stabilization capacitor C1 is down-sized to track line voltage V1 and a reference voltage Vref is modulated by a sine wave to regulate the turn-on intervals under varying supply voltage of the stabilization capacitor C1, which produces an averaged line current resembling a sine wave.
However, if the switch M1 is regarded as a sampler and apply the Sampling Theorem, spectrum of full wave-rectified sine wave can be repeated at the sampling rate. A first ripple frequency of an ac-dc converter is 2 times the line frequency, while the first ripple frequency of a dc-dc converter is the switching frequency. That's a world apart for the requirement of the stabilization capacitor C1.
To counter it, a large electrolytic capacitor C2 is implemented, which will increase the turn-on delay and penalize the cost, size and lifetime.
However, since the LED strings of diode D6 has exponential current-voltage (I-V) characteristics, even small voltage ripples will cause current flowing through the LED strings of diode D6 to fluctuate wildly, making the attempt of suppressing the flickers impossible.
A leading-edge blanking A2 is another concern because any variations in the switch M1, the diode D5, or the inductor T1 can change turn-on spike characteristics, so how to predefine its duration is a guess, in addition to its complexity.